Protein Spin Labeling with a Photocaged Nitroxide Using Diels-Alder Chemistry.

EPR spectroscopy of diamagnetic bio-macromolecules is based on site-directed spin labeling (SDSL). Herein, a novel labeling strategy for proteins is presented. A nitroxide-based spin label has been developed and synthesized that can be ligated to proteins by an inverse-electron-demand Diels-Alder (DAinv ) cycloaddition to genetically encoded noncanonical amino acids. The nitroxide moiety is shielded by a photoremovable protecting group with an attached tetra(ethylene glycol) unit to achieve water solubility. SDSL is demonstrated on two model proteins with the photoactivatable nitroxide for DAinv reaction (PaNDA) label. The strategy features high reaction rates, combined with high selectivity, and the possibility to deprotect the nitroxide in Escherichia coli lysate.

Mapping the helix arrangement of the reconstituted ETR1 ethylene receptor transmembrane domain by EPR spectroscopy.

The plant ethylene receptor ETR1 is a key player in the perception of the phytohormone and subsequent downstream ethylene signal transmission, crucial for processes such as ripening, senescence and abscission. However, to date, there is sparse structural knowledge about the transmembrane sensor domain (TMD) of ETR1 that is responsible for the binding of the plant hormone and initiates the downstream signal transmission. Sequence information and ab initio modelling suggest that the TMD consists of three transmembrane helices. Here, we combined site-directed spin labelling with electron paramagnetic resonance spectroscopy and obtained distance restraints for liposome-reconstituted ETR1_TMD on the orientation and arrangement of the transmembrane helices. We used these data to scrutinize different computational structure predictions of the TMD.

In situ EPR spectroscopy of a bacterial membrane transporter using an expanded genetic code.

The membrane transporter BtuB is site-directedly spin labelled on the surface of living Escherichia coli via Diels-Alder click chemistry of the genetically encoded amino acid SCO-L-lysine. The previously introduced photoactivatable nitroxide PaNDA prevents off-target labelling, is used for distance measurements, and the temporally shifted activation of the nitroxide allows for advanced experimental setups. This study describes significant evolution of Diels-Alder-mediated spin labelling on cellular surfaces and opens up new vistas for the the study of membrane proteins.

Site-directed spin labelling of proteins by Suzuki-Miyaura coupling via a genetically encoded aryliodide amino acid.

We report site-directed protein spin labelling via Suzuki-Miyaura coupling of a nitroxide boronic acid label with the genetically encoded amino acid 4-iodo-l-phenylalanine. The resulting spin label bears a rigid biphenyl linkage with lower flexibility than spin label R1. It is suitable to obtain defined electron paramagnetic resonance distance distributions and to report protein-membrane interactions and conformational transitions of α-synuclein.